There is a related question here, but it's different because that relates to dates whereas I am talking about specific times.
Compare the sentences "The assignment is due by 8:00am" and "The assignment is due before 8:00am".
My native speaker intuition is that Before 8:00am means 7:59:59am is on time, while 8:00:00am and later is late.
Regarding By 8:00am, I think here clearly 8:00:00am is OK and 8:01:00am is not. But what about 8:00:01am or 8:00:59am?